1. Field Of The Invention
This invention relates to inflation packers used in downhole cementing, and more particularly, to an inflation packer having a rupture disc designed to burst at a predetermined pressure to allow cementing above the packer after setting thereof.
2. Description Of The Prior Art
In preparing oil well bore holes for oil and/or gas production, a most important step involves the process of cementing. Basically, oil well cementing is the process of mixing a cement-water slurry and pumping it down through steel casing to critical points located in the annulus around the casing, in the open hole below, or in fractured formations.
Cementing a well protects possible production zones behind the casing against salt water flow and protects the casing against corrosion from subsurface mineral waters and electrolysis from outside. Cementing also eliminates the danger of fresh drinking water and recreational water supply strata from being contaminated by oil or salt water flow through the bore hole from formations containing these substances. It further prevents oil well blowouts and fires caused by high pressure gas zones behind the casing and prevents collapse of the casing from high external pressures which can build up underground.
A cementing operation for protection against the above-described downhole condition is called primary cementing. Secondary cementing includes the cementing processes used in a well during its productive life, such as remedial cementing and repairs to existing cemented areas. The present invention is generally useful in both primary and secondary or remedial cementing.
In the early days of oil field production, when wells were all relatively shallow, cementing was accomplished by flowing the cement slurry down the casing and back up the outside of the casing in the annulus between the casing and the bore hole wall.
As wells were drilled deeper and deeper to locate petroleum reservoirs, it became difficult to successfully cement the entire well from the bottom of the casing, and, therefore, multiple stage cementing was developed to allow the annulus to be cemented in separate stages, beginning at the bottom of the well and working upwardly.
Multiple stage cementing is achieved by placing cementing tools, which are primarily valve ports, in the casing or between joints of casing at one or more locations in the bore hole; flowing cement through the bottom of the casing, up the annulus to the lowest cementing tool in the well; closing off the bottom and opening the cementing tool; and then flowing cement through the cement tool up the annulus to the next upper stage, and repeating this process until all the stages of cementing are completed.
There are cementing applications which necessitate the sealing off of the annulus between the casing string and the wall of the bore hole at one or more positions along the length of the casing string. An example of such an application is when it is desired to achieve cementing between a high pressure gas zone and a lost circulation zone penetrated by the bore hole. Another application is when it is desired to achieve cementing above a lost circulation zone penetrated by the bore hole. A third application occurs when formation pressure of an intermediate zone penetrated by the bore hole is greater than the hydrostatic head of the cement to be placed in the annulus thereabove. Still another application occurs when a second stage of cement is to be placed at a distant point up the hole from the top of the first stage of cement, and a packer is required to help support the cement column in the annulus. A further example of an application for employment of a cementing packer occur when it is desired to achieve full hole cementing of slotted or perforated liners.
An example of such an inflatable packer for cementing is the multiple stage inflatable packer disclosed in U.S. Pat. No. 3,948,322 to Baker, owned by the assignee of the present invention. In this device, an opening plug is dropped into the casing string and pumped down to actuate an opening sleeve to allow inflation of the packer element. A back check valve prevents the packer from deflating. After the packer is inflated, additional pressure is applied which moves an annular valve member to open a port to the well annulus above the inflated packer element. In a later version of this apparatus, a thin walled secondary opening sleeve is sheared to open this port.
Cementing is carried out through the port, and after the cementing operation, a closing plug is dropped into the well casing to actuate a releasing sleeve and move a closing sleeve which seals off the ports. After the operation is complete, the center of the tool may be drilled out, leaving the closing sleeve to permanently seal the ports.
One problem with this apparatus is that the secondary opening sleeve, being essentially a thin walled mandrel, is difficult to manufacture. Further, when the tool is positioned in the well bore, there may be some bending of the tool which can cause the annular valve member or secondary opening sleeve to bind and not open as desired.
The present invention solves this problem by replacing the annular valve member or secondary opening sleeve with a secondary rupture disc which is designed to burst or rupture at the predetermined pressure.